The hydrology and biogeochemistry of low order Precambrian shield catchments

This study examines the effect of landscape heterogeneity on runoff production and material cycling within an upland terrain on the Canadian Precambrian shield. One first order and five zero order shield catchments were hydrochemically monitored in 1988 and 1990. Subcatchments containing the two predominant landscape types found within the study catchments, conifer forested soil islands and open lichen-covered bedrock surfaces, were isolated and studied. Detailed field and laboratory investigations were used to examine the linkages between hydrologic and biogeochemical processes with particular emphasis on aluminum.;Runoff from the study watersheds is initiated as Horton overland flow from open lichen-bedrock surfaces. During larger rain events or under wet antecedent conditions, storage in the forested soil islands is often saturated and runoff is generated as subsurface stormflow and saturated overland flow. Once runoff is generated from the soil islands the entire catchment becomes hydrologically 'linked' and the entire catchment may contribute runoff. During flow recession, discharge is sustained entirely by subsurface stormflow from the forested soil islands. Isotopic hydrograph separation indicates that runoff from lichen-bedrock surfaces is dominated by event water while pre-event water constitutes a larger proportion of soil island runoff.;Temporal trends in runoff chemistry are characterized by initially high concentrations of most elements as material is flushed from dry lichen-bedrock surfaces. Concentrations of NO;Reactive soil aluminum (exchangeable + organic + amorphous forms), comprised ;The papers in this thesis conclude that spatial changes in hydrologic contributing area and temporal changes in flow pathway account for the temporal changes in runoff chemistry observed during this study. The repeating sequence of forested soil islands serves to retard the loss of atmospherically derived and internally weathered material from this landscape. The conclusions reached during this study are a direct result of the experimental approach which allowed the quantification of the hydrochemical fluxes amongst the different landscape elements of the upland Precambrian shield catchments.